Maleimide (MI) and bismaleimide (BMI) functional compounds are useful monomers that have found applications in advanced composite resins and adhesives. They are generally noted for their ability to be polymerized to yield advanced performance resins that possess high glass transition temperatures, high modulus, and good heat resistance properties.
Despite the useful physical properties that can be achieved through the MI and BMI compounds as thermoset monomers, the handling properties of these materials is hampered by their tendency to exist as high melting, crystalline compounds. Only a small handful of BMI monomers are known that are liquid or amorphous at room temperature (see, for example, U.S. Pat. Nos. 3,951,902, 4,564,663 and 6,034,195). The inherent crystalline nature of the vast majority of MI and BMI compounds is a significant impediment against the broad use of these materials in liquid adhesive applications. They cannot be used as the base resin in any liquid adhesive formulation, and at best, can be used only as minor additives to the resin mixture. The use of MI and BMI compounds, even as additives, is further restricted in liquid adhesives to those that can be cured at temperatures greater than or equal to their melting points. Thus, solid MI and BMI compounds cannot generally be used in low temperature cure adhesives.
Some reduction in the melting points of this useful class of compounds can be achieved by melting together two or more of these monomers (see, for example, U.S. Patent Publication No. 20070155869). The value of this melting point suppression technique, however, is very limited since virtually all of the resulting melt blends will still freeze on cooling to yield polycrystalline solids that possess melting points that are well above room temperature. The crystalline MI and BMI compounds may also have poor solubility in other co-monomers. Thus, even if the co-monomers are liquids themselves, the MI and BMI monomers could generally only be added as a dispersion of fine solids in a liquid adhesive. This, in turn, would have a negative impact on the available loading level options for other desirable solid fillers (such as silica, which is often used to depress the CTE of the adhesive composition).
The crystalline properties of the MI and BMI compounds also present difficulties for the formulation of solid adhesives and matrix resins. The high melting points of these compounds, restricts their compatibility with other matrix resins. A physical dispersion of MI and/or BMI solids, for example, in an amorphous co-curative resin could never be as homogeneous as an MI or BMI dissolved in that same amorphous co-curative resin. A need therefore exists for maleimide and bismaleimide compounds that are non-crystalline at room temperature.